This invention relates to well systems particularly adapted for pumpdown operation, and an improved standing valve for use in a retrievable tool string, which cooperates with an improved foot valve, to protect the producing formation from pressure on or flow into during operation of the well.
It is often imperative during well completion and operation, especially if the well is operated using pumpdown or through-flow-line and pumpdown techniques, to protect the producing formation from overpressuring or pumping appreciable volumes into to prevent damage to or kill the formation.
Formation protection is provided by the Well System of U.S. Pat. No. 4,365,671 to Olen R. Long, herein incorporated for reference, but the tool string has been found difficult, if not impossible to pump out and retrieve because of down forces on the tool string being almost equal to up forces developed as pumpout pressure is applied through the H-member cross flow passage between locomotive pistons and the standing valve seal to retrieve the tool string. The locomotive pistons and standing valve seal have very nearly the same sealed area, and pressure introduced between these seals merely places the tool string in tension, and it is not strongly urged to move up tubing and close the foot valve.
Additionally, when the ball valve member in the foot valve of the aforementioned U.S. patent is closed, higher pressures below produce greater upward forces, on the piston sealed in the lower body and on the seat, sealed in the piston, tending to crush the valve ball member. These upward forces also press the seat on the ball, greatly increasing rubbing friction forces which induce high stresses in the ball rotating mechanism when opening the foot valve. If these upward forces become great enough, the shear pin positioning the piston on the seat may be prematurely sheared and fluids may inadvertently bypass the closed foot valve and flow into the formation below, with severely damaging results.